When I have asked, they say take the kids shortly after they get teeth. When talking to the pediatric dentist that we go to, he is seeing kids that at age 4 have many (like 8+) cavities. So much is dependent on eating habits and brushing.
That said, I have taken my kids starting at about a year. (Sounds crazy, I know.) When they are that small, they can quickly clean the child's teeth - my kids have always tolerated this without any problems. They have me sit knee to knee with the hygienist/dentist, and hold the baby, then lean the baby back into the other lap. The baby has been totally fine, as mom is right there, and they (the hygienist/dentist) can get to the teeth. The whole experience is pretty quick - especially because they don't have that many teeth.
By age 2 my kids have been OK sitting up in the 'big' (but child size) chair on their own. They have not done fluoride until 2.5 or so.
They really want the kids to have a positive view of the dentist, so no matter the age, the first few cleanings are going to be minimally invasive and VERY positive.
My dentist has never recommended anything other than what I am already doing. (We brush teeth and floss nightly, no food/drink after that.) My 13month old we brush his teeth occasionally, but not nightly yet. (To be fair, he only has 2 teeth, so it is not too big of a deal.)
We have never had a problem at the dentist, or with cavities.
I asked how the kids can have that many cavities at such a young age, and he said that they can be drinking cokes and sodas and chocolate milk all the time. That much sugar and other acids on their teeth will cause problems. (He had a child show up for the post 10 cavities filling check, carrying a bottle of coke with him.... at age 4.... hard to believe.)
If you are avoiding those things already, then older is probably fine.
(I have bad teeth, despite high amount of dental self care, and appts.... so I am trying to get the kids off to a better start.... That and wanting them to be comfortable with the dentist is why we start early.
I recently had an appt and had to spend the day before listening to the kids complain that *I* had an appt, why didn't THEY have an appt! Odd kids, I suppose, but it works for us. :)
At 18 months, they can work with the child. It is not like an adult cleaning, no scraping.... Just brush, floss, polish... it is really fast, if they are good. :)
Hope that helps... sorry it is so long. :)